The human alpha globin gene has been shown to be on chromosome 16 and the human beta globin gene to be on chromosome 11. These results were obtained by direct analysis of the globin gene content of mouse X human fibroblasts hybrid cells containing a variable complement of human chromosomes. Because LDH-A is on chromosome 11 and therefore linked to the human beta globin locus, it can be used as a marker to screen hybrid cells for the presence of the beta globin gene. Among many different hybrids formed by fusion of mouse erythroleukemia cells and human fibroblasts, several were identified which contained chromosome 11. Totally pure human alpha and beta complementary DNAs were prepared from recombinant plasmids containing the human globin genes. Analysis of RNA from hybrid cells known to contain chromosome 11 which had been induced to undergo erythroid differentiation by growth in dimethylsulfoxide, led to the detection of human beta globin mRNA sequences.